The present invention relates to an electric motor and more particularly relates to the structure of a small and flat-type electric motor and the production method thereof; wherein the permanent magnet segments are so formed and so arranged as to prevent the abrupt change in the distribution of magnetic flux which may otherwise be caused each time the armature segments travel past the electric position, to thereby smoothly rotate the motor without variations in the rotation torque. Moreover a groove provided between the permanent magnet segments may be effectively utilized and the motor may be easily assembled.
Recently the small and disk-shaped electric motors have been widely used in toys, portable stereo cassette tape recorders, small-type audio systems, cameras, etc. As these devices are required to be further miniaturized and to have a higher performance, the small and disk-shaped electric motors have been accordingly required to be smaller and also to be flat to the maximum extent with a high property being maintained.
Now the conventional small and disk-shaped electric motors have a plurality of permanent magnet segments arranged in a same plane opposite to a plurality of armature segments with a common center axis to provide therebetween a magnetic field, wherein the permanent magnet segments are divided from each other by the geometrical lines which may be parallel with a normal extended from the center rotation axis of the armature segments. In other words the dividing lines of the permanent magnet segments are extended in the radial direction. Therefore the armature segments subject to the shock due to the abrupt change in the distribution of magnetic flux which is caused by the permanent magnet segments each time the armature segments pass the electric positions, and as the result the torque varies affecting the rotation of the motor.
Further the conventional small and disk-shaped electric motors have a commutator and brushes to be contacted to the commutator provided on one side of the armature segments, on the opposite side of which are arranged the permanent magnet segments with an air gap provided therebetween. It has therefore been required to provide a space between the armature segments and the motor case to accommodate therein the commutator and the brushes. Such an arrangement of elements has prevented the motor from being made smaller and more flat. Further the conventional commutator has the commutator segments arranged in a cylindrical form around the center rotation axis of the armature segments. It has therefore been required to assemble the commutator and brushes under the condition that the elastic brush carriers are kept pressed. Thus the production process has required an elaborating and time consuming work.
Further the conventional small and disk-shaped electric mortors have been of a moving-coil type instead of a slot-type in which the winding of coil is rather difficult and also the weight of rotor increases, because the moving-coil type has the properties of little rotational variations and of low inductance. However actually the motor of moving-coil type has an armature segments generally providing a low density of magnetic flux to be produced. This means that it is difficult to obtain a desired torque by means of a ferrite permanent magnet segments in case of the extremely flat motor. Therefore in order to obtain a desired torque, it is required to employ a samarium cobalt permanent magnet segments which are extremely expensive, and accordingly the cost of the motor becomes higher.
Further the conventional small and disk-shaped electric motors have generally an iron case constituting a part of magnetic circuit together with the permanent magnet segments and the armature segments so as to produce a magnetic field in which the armature segments may be rotated. In this case, in order to increase the torque of the motor, it is required to make the two air gaps small as much as possible which are to be provided between the armature segments and the permanent magnet segments and between the armature segments and the motor case. However for satisfying this requirements, the motor case must be formed with an extremely high precision. Actually it has been difficult to provide a desired air gap between the armature segments and the motor case in the small and flat-type electric motor, and it has been unavoidable to set the air gap bigger than that to be desired. Thus there has been a limitation in setting a maximum possible output of the motor. Moreover the motor case has been made of a magnetic material such as iron which has to be precisely processed, for example, by way pressing, and accordingly the production cost increases.
Still further the conventional small and disk-shaped electric motors have been of a moving-coil type instead of a slot-type which requires a rather complex winding of coil resulting in the difficulty of reducing the size of the armature segments, because the moving-coil type is suitable for reducing the size as well as the weight of the armature segments. However the armature coils of the moving-coil type motor are axially hollow and therefore the coercive force is smaller and also the density of magnetic flux is lower. It has therefore been difficult to obtain a desired torque. For obtaining a desired torque, it has been required to employ a samarium cobalt permanent magnet segments which have a high density of magnetic flux, but which are extremely expensive. An experiment has been made in combination of the ferrite permanent magnet segments and making up an extremely flat motor of less than the thickness of 6 mm and of three armature segments with bipolar electrodes. The experiment has proved that such a motor will not produce a torque which may be practically available. Then the inventor of this application has had a thought of inserting an iron core into each of the axially hollow armature segments so as to concentrate the magnetic flux to the axial part of the armature segments. Moreover the inventor has thought it better to make the iron core by laminating a leaf plate to more heighten the coercive force.